spi: dt-bindings: document zero value for spi-{rx,tx}-bus-width properties

Following a change to the SPI framework, providing a value of zero for
'spi-rx-bus-width' and 'spi-tx-bus-width' is now possible and will
essentially mean that no RX or TX is allowed.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201221152936.53873-3-alexandru.ardelean@analog.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -152,8 +152,9 @@ patternProperties:
       spi-rx-bus-width:
         description:
           Bus width to the SPI bus used for read transfers.
+          If 0 is provided, then no RX will be possible on this device.
         $ref: /schemas/types.yaml#/definitions/uint32
-        enum: [1, 2, 4, 8]
+        enum: [0, 1, 2, 4, 8]
         default: 1
 
       spi-rx-delay-us:
@@ -163,8 +164,9 @@ patternProperties:
       spi-tx-bus-width:
         description:
           Bus width to the SPI bus used for write transfers.
+          If 0 is provided, then no TX will be possible on this device.
         $ref: /schemas/types.yaml#/definitions/uint32
-        enum: [1, 2, 4, 8]
+        enum: [0, 1, 2, 4, 8]
         default: 1
 
       spi-tx-delay-us: